The present invention relates to a coil for circulation of coolant fluid and to a method for manufacturing such a coil. The invention also concerns a reactor for the treatment of a viscous medium or for carrying out chemical reactions in a viscous medium, such as a polymerization, such a reactor comprising such a coil. Finally, the invention concerns the use of such a reactor.
It is known to effect polymerization of polyamide continuously or discontinuously. In the so-called discontinuous methods, one proceeds by batches with reactors of autoclave type. In that case, it is known to evaporate the water of an aqueous solution of two monomers and the water produced by their polymerization, thanks to an outside heat supply.
Such heat supply must be sufficient for the reaction of polymerization to take place in a period of time compatible with the criteria of productivity in force in the industrial world. The heat supply must not be too great in order to avoid, as far as possible, entraining one of the monomers with the vapour phase. If a quantity of monomers is entrained with the vapour phase, this quantity must be constant in order that the characteristics of the polyamide obtained be reproducible. Furthermore, the supply of heat makes it possible to monitor the reaction of polymerization insofar as it allows the evaporation of the water to be controlled.
To supply heat to the reaction medium, coils in which a coolant fluid circulates have been used in low-capacity autoclaves, i.e. ones with a volume less than 3 m3.
For reactors of greater capacity, in particular of the order of 5 to 6 m3, it has been possible to envisage using a coil and an agitator, the latter aiming at improving the homogeneity of the reaction medium and at increasing the coefficient of heat transfer.
However, this solution cannot be applied to high-capacity reactors, in particular ones with a capacity greater than 8 m3, as it is impossible to sufficiently increase the exchange surfaces constituted by the walls of the coil. In effect, if the overall diameter of the coil is increased, it is in that case no longer possible to house an efficient agitator in the vessel of the reactor. If the diameter of the tubes constituting the coil is decreased, the pressure drops associated with the circulation of coolant fluid in these tubes increase significantly. If a coil of very elaborate shape is created, the axial re-circulation of the reaction medium is hindered and the so-called “pumping” effect at the centre of the agitator is cancelled. Finally, a coil of elaborate shape with tubes of small diameter would not satisfy the criteria of mechanical strength allowing it to withstand prolonged use and/or incidents in manufacture.
It is a more particular object of the present invention to overcome these drawbacks by proposing a novel coil which allows an efficient heat supply to a reaction medium of large volume, while being compatible with the dimensions of a reactor vessel and with an agitator.